iStockphoto/Thinkstock(EAGLE PASS, Texas) -- The director and 15 other staffers of a Mexican prison on the Texas border are being held under house arrest while authorities investigate whether they helped Mexico's most violent drug cartel, the Zetas, break more than 130 inmates out of the prison during a daring daylight escape Monday.

Initial media reports said prisoners had escaped through a 21-foot tunnel equipped with wires and cables after overpowering guards at the prison in Piedras Negras, just across the U.S. border from Eagle Pass, Texas. But top officials in the Mexican state of Coahuila were always suspicious that guards had been involved in the escape, and interrogation of three recaptured escapees revealed that many prisoners had walked straight out the front gate into waiting trucks.

"[The 3 inmates] were recruited by that group [the Zetas], which is waging a war in Tamaulipas and other states of the republic," Jorge Luis Moran, public safety secretary of Coahuila, said on a Mexican television network, Forotv.

Moran told another news channel that not all the detainees had escaped via the tunnel. The prison's director, Miguel Angel Resendiz, was fired after the escape.

The Zetas were started by members of the Mexican military who went to work as security for the Gulf Cartel and then formed their own competing drug-smuggling operation, which soon became dominant in southeast and eastern Mexico. The Zetas are famous for brutality, including mass killings and beheadings.

The Zetas have been battling the Sinaloa cartel, the country's most powerful cartel, for control of Coahuila and other territory along the Texas border.

Moran said that on Monday, police responding to the prison escape were attacked by gunmen with grenades and rifles. Four of the gunmen were killed; one of the men was believed to be a Zeta attempting to prevent the police from searching for escapees.

In an emailed statement, U.S. Customs and Border Protection said it had been made aware of the escape and was in touch with Mexican officials.

"CBP is aware of the reported jail break in Northern Mexico, and out of an abundance of caution, has placed its officers and agents in the Eagle Pass, Texas area on alert," said spokesman Dennis Smith. "At this point, CBP has no reports of escapees attempting to cross the border."

"We remain in communication with our law enforcement partners in Mexico and maintain a shared interest in keeping our mutual border secure," said Smith.

In 2010, more than 150 inmates broke out of a prison in Nuevo Laredo, across the Rio Grande from Laredo, Texas. Forty-one guards were charged with aiding in that escape, the largest in Mexico in recent years.